This invention relates in general to the transmission of varying information from remote locations and, more particularly, to an electro optic system for transducing a voltage signal into an optical signal, transmitting the signal via an optical fiber to a distant location and interpreting the signal.
Often it is necessary to sense varying information such as voltage or current in an electrical power system or temperature or pressure in a hydraulic system for use at a distant location. The sensing location may be hostile due to high temperatures or other severe environmental conditions. Or the location may be inaccessible so that direct observation of the varying condition cannot be accomplished such as in the measurement of aircraft engine or control surface conditions. In the past a variety of mechanical, hydraulic and electrical arrangements have been used to transmit information from remote sensors to central control stations, such as an aircraft cockpit, the bridge of a ship or a power station control room. While these systems are often effective, they are generally heavy, complex and subject to physical damage.
Electrical sensing and information transmission systems are widely used due to their light weight, ease of assembly and ability to provide convenient redundant signal paths to protect against signal loss due to damage to one wire. However, these systems are subject to power failures, short circuits in the wires, electromagnetic interference (EMI) from nearby wiring or electrical devices and are potentially subject to destruction by electromagnetic pulses (EMP) from nuclear explosions or other sources. There is a particular need to overcome these problems in military aircraft, missiles and ships and in automated machine tools and robotics where electromagnetic interference poses serious problems.
Recently, considerable interest has developed in using optical fiber systems for remote sensing and control applications. Optical fibers have many of the advantages of the electrical wired systems while being impervious to electrical short circuits, EMI and EMP. A typical fiber optic temperature or pressure sensing system as disclosed by Anderson et al. in U.S. Pat. No. 4,672,199 uses a mirror on a temperature responsive bimetallic strip or a pressure responsive piston to vary the light transmitted through a fiber to a light sensor. The system measures only temperature quantity and cannot transmit amplitude and wave form information from a sensed electrical signal. Other systems which sense variations in an optical property, such as light intensity, and transmit a corresponding light signal through an optical fiber to a detector are described by Adolfsson et al. in U.S. Pat. Nos. 4,417,140 and 4,433,238 and by Brogardh in U.S. Pat. No. 4,531,230. While each of these disclosed fiber optic sensing systems is effective for the limited purposes described, they cannot transduce very low power level electrical signals into optical signals without electronic circuitry or supplemental electrical power at the transmitter location and do not transmit a signal providing sensed voltage amplitude and wave form information.
Thus, there is a continuing need for simple, accurate, remote sensing systems which are capable of transducing low voltage/low power electrical signal frequency, amplitude and waveform information with a transmitter which can operate in a harsh environment without electronic components or supplemental electrical power.